This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Today, most customer homes receive a number of programs containing video and audio content, as well as a large amount of data, from a plurality of sources, such as broadcast television, cable, satellite, digital subscriber line systems. These systems often utilize distribution networks to deliver the programs and content to the customer premises. Many distribution networks carry content on multiple channels or transponders that may originate from different sources (e.g. multiple satellites, cable head-ends, etc). The multiple channels or transponders from different sources are brought together on a single medium (e.g. coaxial cable) before going, for example, into a customer's home for connection to a television or settop box. As a result, a frequency map identifying the incoming channels or transponders, as well as certain characteristics, such as modulation formats, of each channel or transponder may not be completely known initially.
In instances where the frequency map is not completely known, and the modulation formats for the channels or transponders are not otherwise identified or specified, the television or settop box may be designed to perform some form of search in order to identify the modulation formats for channels or transponders so that the television or settop box can proceed to demodulate and decode the received signal. In one method, the television or settop box may implement a trial and error modulation format identification search. In a trial and error search, the receiving circuit in the television or settop box initializes the link circuit to demodulate the signal based on a first modulation format. If demodulation is unsuccessful, the search continues as each possible format may be iteratively attempted, to determine, for instance, the modulation depth or mapping of the constellation of symbols in the signal format.
Modern broadcast communications signal may use constellation maps that include formats having as few as 2 symbols or as many as 256 symbols and locations. For instance, there are several different constellations that can be transmitted under most QAM-based systems (e.g.—digital video broadcast-cable (DVB-C), or US cable standard J.83A). The trial and error modulation format identification search may become time consuming and inefficient if the first modulation format chosen is very different from the modulation format used by the received signal.
The problems with the trial and error search approach are further exacerbated by a system containing a large multi-source network with many possible signal types. The approach can take an unacceptably long period of time to determine the modulation format of each channel or transponder in such a large multi-source network. Further, if the network requires frequent re-initialization due to changes in the frequency mapping as well as changing to the modulation format of the channels or transponders, the resulting period of waiting may be unacceptable to the user. Therefore, there is a need for an improved apparatus and method for determining the modulation format of a signal.